Permanent magnets with tailored texture and magnetic orientation

ABSTRACT

Some variations provide a permanent-magnet structure comprising: a region having a plurality of magnetic domains and a region-average magnetic axis, wherein each of the magnetic domains has a domain magnetic axis that is substantially aligned with the region-average magnetic axis, and wherein the plurality of magnetic domains is characterized by an average magnetic domain size. Within the region, there is a plurality of metal-containing grains characterized by an average grain size, and each of the magnetic domains has a domain easy axis that is dictated by a crystallographic texture of the metal-containing grains. The region has a region-average easy axis based on the average value of the domain easy axis within that region. The region-average magnetic axis and the region-average easy axis form a region-average alignment angle that has a standard deviation less than 30° within the plurality of magnetic domains. Many permanent-magnet structures are disclosed herein.

PRIORITY DATA

This patent application is a non-provisional application claimingpriority to U.S. Provisional Patent App. No. 63/061,798, filed on Aug.6, 2020, and to U.S. Provisional Patent App. No. 63/061,800, filed onAug. 6, 2020, each of which is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to permanent magnets withtailored magnetic properties.

BACKGROUND OF THE INVENTION

A magnet is a material or object that produces a magnetic field. Themagnetic field creates a force that acts on other magnets orferromagnetic materials, such as iron, steel, nickel, or cobalt. Apermanent magnet is an object made from a material that is magnetizedand creates its own persistent magnetic field.

Although ferromagnetic and ferrimagnetic materials are the onlymaterials attracted to a magnet strongly enough to be commonlyconsidered magnetic, all substances respond at least weakly to amagnetic field. Some ferromagnetic materials are magnetically softmaterials (“soft magnets”) such as annealed iron. Soft magnets can bemagnetized but do not tend to stay magnetized. On the other hand,magnetically hard materials (“hard magnets”) tend to stay magnetized andare typically difficult to demagnetize. Permanent magnets are commonlymade from hard ferromagnetic materials such as alnico alloys and ferritethat are subjected to special processing in a strong magnetic fieldduring manufacture to align the internal microcrystalline structure,making the materials very hard to demagnetize. Demagnetizing a saturatedmagnet requires application of a magnetic field whose minimum strengthcorrelates with the magnetic coercivity of the magnet. Hard magnets havehigh magnetic coercivity, while soft magnets have relatively lowmagnetic coercivity.

Permanent magnets are commonly made from neodymium, alnico alloys, orferrites. Neodymium magnets are the strongest and most expensive of thethree materials. Strong permanent magnets—especially sintered neodymium(Nd) magnets—have had their magnetic domains oriented in the directionof an easy axis to maximize magnetic strength.

Applications of permanent magnets include, but are not limited to,electric vehicle motors, electric take-off assist motors and sensors,magnetic separators, and magnetic detectors.

State-of-the art bulk permanent magnets are conventionally produced bydie-press and sintering methods in which consolidation of powderprecursors followed by heat treatment in a magnetic field produces amagnetic and crystallographic alignment in one specific orientationthroughout the magnet. Die-press and sintering methods are inherentlylimited in possible geometries by the shape of the die (prismaticgeometry) and eventual loss of net shape due to a shrinkage duringsintering. Additionally, the mass loss costs incurred by machining intoa final net shape significantly increase the cost of magnets. Forexample, magnet material cost is about 70% of an electric motor and is aprimary limiting factor to wide-scale adoption of electric-motorautomotive vehicles powered by permanent-magnet motors. Material cost ofrare-earth permanent-magnet materials, required for high-performanceautomotive and aerospace platforms, is a substantial commercial problemtoday.

Permanent magnets can be optimized by locally tailoring crystallographictexture in regions susceptible to demagnetization by tailoring theorientation of the easy axis. Conventionally processed high-performingpermanent magnets produce a single, or narrow distribution of, easy axisorientations and magnetizations (see FIG. 1). Permanent magnets withthis anisotropic crystallographic texture (e.g., die-pressed NdFeB ordirectionally solidified AlNiCo or FeCoCr) demonstrate desirablemagnetic performance, such as magnetic energy density, in comparison toisotropic variants. These anisotropic materials are conventionallyproduced by die compaction where consolidation of powder precursorsproduces a uniform texture, a result of directionally imposed plasticdeformation, throughout the material. The nearly uniform orientation ofthe crystal structure aligns the easy axis of each grain, therebyallowing the material to be magnetized in a single orientation with anarrow distribution, producing larger achievable magnetizations.However, in many applications such as electric motors, generatedmagnetic fields interact non-uniformly with these magnetic materials.Because fields generated in these applications are non-uniformlyconcentrated in regions of high and low magnetic flux density tomaximize motor efficiencies, regions such as corners and surfaces of thepermanent magnet material are more susceptible to demagnetization thanthe interior bulk. The magnetic field angle of incidence in theseregions can vary significantly away from the angles producing maximaltorque and lead to demagnetization under weaker applied fields—thuslimiting the achievable weight and volume efficiencies of the magneticmaterial. State-of-the-art manufacturing die-press and sinter methodsare significantly constrained to uniaxial textures and, as mentionedabove, prismatic geometries. Many desirable textures and magnetic shapesare not possible using the known art.

It would be desirable to impose different magnetic orientations atspecific locations within a magnet, to augment the capability oftailoring a magnetic field. For instance, field strength could beincreased on one side of a magnet while cancelling the field to nearzero on the other side of the magnet, using arrays of magneticorientations in different directions. These types of designs aretypically assembled by bonding magnets together, in a structure known asa Halbach array. More generally, location-specific magnetic orientationswould be beneficial because the shape and intensity of the magneticfield generated by a permanent magnet could be designed into the magnetarchitecture without physically changing the shape of the magnet.

Magnetic anisotropy describes how an object's magnetic properties can bedifferent depending on direction. When there is no preferentialdirection for an object's magnetic moment, the object will respond to anapplied magnetic field in the same way, regardless of which directionthe field is applied. This is known as magnetic isotropy. In contrast,magnetically anisotropic materials will be easier or harder to magnetizedepending on which way the object is rotated. For many magneticallyanisotropic materials, there are at least two easiest directions tomagnetize the material, which are a 180° rotation apart. The lineparallel to these directions is called the magnetic easy axis and is anenergetically favorable direction of spontaneous magnetization.

Magnetocrystalline anisotropy has a great influence on industrial usesof ferromagnetic materials. Materials with high magnetic anisotropyusually have high magnetic coercivity—that is, they are hard todemagnetize. These are called hard ferromagnetic materials and are usedto make permanent magnets. For example, the high anisotropy ofrare-earth metals is mainly responsible for the strength of rare-earthmagnets. During manufacture of magnets, a powerful magnetic field alignsthe microcrystalline grains of the metal such that their easy axes ofmagnetization all point in the same direction, freezing a strongmagnetic field into the material.

Nearly uniform orientation of a crystal structure aligns the easy axisof each grain, allowing the material to be easily magnetized with asmall orientation distribution and giving the material a high resistanceto uniform demagnetizing fields. Resistance to demagnetization has beenincreased in the art by manipulating process-dependent microstructureand chemistry to optimize competing mechanisms in generating high-energyproducts. However, in applications such as electric motors, generatedmagnetic fields interact non-uniformly with magnetic materials. Becausefields generated are non-uniformly concentrated to regions of high andlow magnetic flux density, regions such as corners and surfaces of themagnet are highly susceptible to demagnetization. In addition, thecorners are inherently susceptible to demagnetization even in withuniform flux, due to the magnet geometry and microstructure. Themagnetic field angle of incidence in these regions can varysignificantly away from the angles producing maximal torque and lead todemagnetization, thereby limiting the achievable weight and volumeefficiencies of the magnetic material.

State-of-the-art bulk permanent magnet materials (e.g. NdFeB) withanisotropic crystallographic texture have desirable magnetic performancein comparison to isotropic variants. Anisotropic materials areconventionally produced by die-press and sintering methods in whichconsolidation of powder precursors produces a relatively uniform texturethroughout the material. Heat treatment in a magnetic field thenproduces relatively uniform magnetic alignment of all grains in thematerial. The die-press and sinter methods are significantly constrainedto prismatic geometries and uniaxial textures, and therefore limited intheir ability to achieve desirable crystallographic textures andmagnetic shapes.

The benefits of easy axis alignment through texturing in permanentmagnets are well-known. See Dulis et al., “Solid NdFeB Magnets Made byGas Atomization and Extrusion”, Science and Technology of NanostructuredMagnetic Materials, 1991, pages 599-606; and White et al., “Net shapeprocessing of alnico magnets by additive manufacturing”, IEEETransactions on Magnetics, 53.11 (2017): 1-6. Some methods of texturecontrol in NdFeB are based on plastic deformation of a consolidatedmaterial. In this technique, the crystallographic orientation is largelyuniform in the extrusion direction (e.g. c-axis for NdFeB permanentmagnets) with limited ability to control the texture in orientationsother than the direction of plastic deformation. Directionalsolidification is alternatively used to produce uniaxial texture, buttexture in these methods is uniaxial and dependent on the maximumthermal gradient. Directional solidification is severely limited in twoways. First, texture in the preferred growth direction, in the case ofNdFeB, produces a preferred growth orientation [100] orthogonal to theeasy axis direction which is the [001] c-axis in NdFeB. Second, thesemethods produce long columnar grains well above the single-domain limitfor these materials, thus limiting the achievable coercivity. See Ma etal., “The impact of the directional solidification on the magneticproperties of NdFeB magnets”, Journal of applied physics 70.10 (1991):6471-6473.

Current methods to additively manufacture permanent magnets are limitedto those easily processible by liquid-processing methods and verylimited in achievable performance for higher-energy-product NdFeBmaterials where microstructures are heavily optimized for die-pressmethods. See Kolb et al., “Laser Beam Melting of NdFeB for theproduction of rare-earth magnets”, 2016 6th International ElectricDrives Production Conference (EDPC), IEEE, 2016 and Jacimovic et al.,“Net shape 3D printed NdFeB permanent magnet”, preprint arXiv:1611.05332[physics.ins-det] (2016).

Control of crystallographic texture in directed energy deposition (DED)manufacturing using external magnetic fields to alter crystal texture instructural alloys has been described in academic studies. See Wang etal., “Texture control of Inconel 718 superalloy in laser additivemanufacturing by an external magnetic field”, Journal of materialsscience 54.13 (2019): 9809-9823; and Wang et al., “Effect of ExternalMagnetic Field on the Microstructure of 316L Stainless Steel Fabricatedby Directed Energy Deposition”, Proceedings of the ASME 2019International Mechanical Engineering Congress and Exposition, Volume 2B:Advanced Manufacturing (2019). While DED using powder spray or wiresproduces parts with less geometric constraints than traditional methods,these parts usually need extensive finishing procedures due to a poorsurface finish after deposition has occurred. This is in contrast topowder bed-based processes which can produce near-net-shape parts afterdeposition.

The current methods are inherently limited to microstructuresconventionally achieved in the unique thermoprocessing conditions ofadditive manufacturing (columnar grains >100 μm), which limits themagnetic performance of additively manufactured materials. In addition,current additive-manufacturing methods to produce hard magneticmaterials are limited to materials with low-energy products (<50 kJ/m³).State-of-art NdFeB magnet materials can have energy products >400 kJ/m³,but only when employing conventional processing methods that lacktexture control.

There is a desire to control solidification of additively manufacturedor welded microstructures on length scales lower than provided by theprior art, to maximize the resistance to demagnetization in addition tocontrolling the orientation of the magnetic easy axis. It is sought tobridge extensive gap in performance between additive manufacturing andconventional processing methods. New or improved methods, structures,and systems are desired for tailoring magnetism in permanent magnets.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, aswill now be summarized and then further described in detail below.

Some variations provide a permanent-magnet structure comprising:

a region having a plurality of magnetic domains and a region-averagemagnetic axis, wherein each of the magnetic domains has a domainmagnetic axis, wherein each domain magnetic axis is substantiallyaligned with the region-average magnetic axis, and wherein the pluralityof magnetic domains is characterized by an average magnetic domain size;and

within the region, a plurality of metal-containing grains, wherein theplurality of metal-containing grains is characterized by an averagegrain size,

wherein each of the magnetic domains has a domain easy axis that isdictated by a crystallographic texture of the metal-containing grains;

wherein the region has a region-average easy axis based on average valueof the domain easy axis within the region; and

wherein the region-average magnetic axis and the region-average easyaxis form a region-average alignment angle θ that preferably has a θstandard deviation of less than 30° based on alignment-angle variancewithin the plurality of magnetic domains.

There may be one region or many regions. When there are multipleregions, the individual region-average magnetic axes and the individualregion-average easy axes may vary spatially, such as differentorientations in different corners of the structure. Regions may be bulkregions contained in the interior of the permanent-magnet structureand/or surface regions contained at the surface of the permanent-magnetstructure.

The magnetic domain averages and easy axis averages may varyspatially—e.g., different orientations in different corners—in differentareas of the permanent-magnet structure. The variations across regionsmay be regular or irregular.

The metal-containing grains may contain a metal selected from the groupconsisting of Fe, Ni, Al, Co, Cr, Nd, B, Sm, Dy, and combinationsthereof. In some embodiments, the metal-containing grains contain ametal alloy selected from the group consisting of NdFeB, FeCoCr,FeAlNiCo, SmCo, Dy₂O₃, SrRuO₃, and combinations thereof. When there aremultiple regions, there may be different compositions in those regions,including different types or amounts of metal-containing grains, forexample. An example is a surface region with a different compositionthan a bulk region of the permanent-magnet structure.

In some embodiments, the region-average alignment angle θ is from −10°to 10°, from −5° to 5°, from −2° to 2°, or from −1° to 1°. In otherembodiments, the region-average alignment angle θ is selected from 0° to90°. In certain embodiments, the region-average alignment angle θ is 0°.In certain embodiments, the region-average alignment angle θ is 90°. Theregion-average alignment angle θ standard deviation may be less than20°, less than 10°, or less than 5°, for example.

In some embodiments, the region-average easy axis has a standarddeviation that is less than 25°, less than 20°, less than 10°, or lessthan 5°. This standard deviation is calculated based on all of thedomain easy axes within a given region. In certain embodiments, eachdomain easy axis is substantially aligned with the region-average easyaxis, in which case the standard deviation may be less than 2°, lessthan 1°, less than 0.5°, less than 0.1°, or about 0°.

A magnetic domain may contain an individual metal-containing grain.Typically, a magnetic domain contains multiple metal-containing grains.In some embodiments, the average magnetic domain size is about the sameas the average grain size. In other embodiments, the average magneticdomain size is larger than the average grain size.

The average magnetic domain size may be from 1 micron to 1000 microns,for example. In some embodiments, the average magnetic domain size isfrom 10 microns to 10 millimeters.

The average grain size may be from 0.1 microns to 50 microns, forexample. An exemplary average grain size is from about 1 micron to about5 microns.

In some embodiments, the metal-containing grains are substantiallyequiaxed grains. In other embodiments, the metal-containing grains aresubstantially columnar grains. In certain embodiments, themetal-containing grains are a combination of substantially equiaxedgrains and substantially columnar grains.

In the permanent-magnet structure, the region may have a characteristiclength scale selected from 100 microns to 1 meter, for example.

In some permanent-magnet structures, there is at least one additionalregion having a plurality of additional magnetic domains and a pluralityof metal-containing grains. The additional region may have a differentcomposition compared to the bulk region(s).

In some embodiments, the additional region is contained at a corner oredge of the permanent-magnet structure. In these or other embodiments,the additional region is contained at a surface of the permanent-magnetstructure.

In some embodiments, the permanent-magnet structure is contained withina Halbach array.

In some embodiments, the permanent-magnet structure is an additivelymanufactured structure.

In some embodiments, the permanent-magnet structure is a weldedstructure.

The permanent-magnet structure may be contained within a solid bulkmagnet. Alternatively, the permanent-magnet structure may be containedwithin a porous magnet.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 depicts a conventional isotropic magnet (left side) and aconventional anisotropic magnet (right side).

FIG. 2 depicts a magnet with a magnetic field applied in three differentdirections within three regions.

FIG. 3 depicts a magnet with a magnetic field in three differentdirections in three regions, with the crystallographic orientationcontrolled through thermal gradients in solidification.

FIG. 4 shows an exemplary permanent magnet with a shaped field due totailored magnetization, with north (N)-south (S) axis orientation havingangle θ from normal direction.

FIG. 5 depicts several examples of magnetic configurations for differentorientations that may be employed in various embodiments.

FIG. 6 depicts a Halbach array configured to cause cancellation ofmagnetic components, resulting in a one-sided magnetic flux.

FIG. 7 shows photomicrographs of the Nd₂Fe₁₄B magnet microstructure witha shifted dendrite growth direction against the direction of maximumthermal gradient, in the Example.

FIG. 8 shows a photomicrograph top view of the laser-welded Nd₂Fe₁₄Bmagnet structure, in the Example.

FIG. 9 shows the increase in NdFeB easy axis [001] texture along thescan vector direction in IPF density plots, in the Example. EBSD mapsare provided for reference orientations.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The magnets, structures, compositions, methods, and systems of thepresent invention will be described in detail by reference to variousnon-limiting embodiments.

This description will enable one skilled in the art to make and use theinvention, and it describes several embodiments, adaptations,variations, alternatives, and uses of the invention. These and otherembodiments, features, and advantages of the present invention willbecome more apparent to those skilled in the art when taken withreference to the following detailed description of the invention inconjunction with the accompanying drawings.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly indicates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as is commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Unless otherwise indicated, all numbers expressing conditions,concentrations, dimensions, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending at least upona specific analytical technique.

The term “comprising,” which is synonymous with “including,”“containing,” or “characterized by” is inclusive or open-ended and doesnot exclude additional, unrecited elements or method steps. “Comprising”is a term of art used in claim language which means that the named claimelements are essential, but other claim elements may be added and stillform a construct within the scope of the claim.

As used herein, the phrase “consisting of” excludes any element, step,or ingredient not specified in the claim. When the phrase “consists of”(or variations thereof) appears in a clause of the body of a claim,rather than immediately following the preamble, it limits only theelement set forth in that clause; other elements are not excluded fromthe claim as a whole. As used herein, the phrase “consisting essentiallyof” limits the scope of a claim to the specified elements or methodsteps, plus those that do not materially affect the basis and novelcharacteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consistingessentially of,” where one of these three terms is used herein, thepresently disclosed and claimed subject matter may include the use ofeither of the other two terms, except when used in Markush groups. Thusin some embodiments not otherwise explicitly recited, any instance of“comprising” may be replaced by “consisting of” or, alternatively, by“consisting essentially of.”

The present invention provides a permanent magnet with location-specificmagnetic orientation and crystallographic texture. This disclosuredescribes the structure of permanent magnets with tailored texture aswell as methods of controlling solidification to achieve a tailoredtexture.

The methods disclosed herein provide an efficient way to shape themagnetic field induced by a permanent magnet. In some variations,additive manufacturing (e.g., selective laser melting or electron beammelting) is employed to fabricate a permanent magnet voxel by voxel andlayer by layer, so that the magnetic orientation of each voxel may beindependently aligned via applying a magnetic field during thesolidification of each voxel. A “voxel” is a volumetric (3D) pixel. Inadditive manufacturing, there is solidification of individual voxels sothat the magnetic field may be varied voxel-by-voxel if desired. Aplurality of voxels forms a single layer having a thickness defined bythe voxel height.

Additive manufacturing also enables site-specific control of thecrystallographic orientation during three-dimensional (3D) printing. Thesolidification texture depends on crystal structure, lattice strain, andsurface attachment kinetics, and is directionally dictated by a maximalthermal gradient during the phase transformation from liquid to solid.Thermal gradients may be controlled during selective laser melting using(a) a laser scan strategy, e.g. locally heating in a predeterminedraster pattern and energy intensity and/or (b) an externally appliedmagnetic field to influence texture evolution. In some embodiments, themagnetic easy axis orientation aligns with a crystallographicorientation in which resistance to demagnetization is maximized. Usingthe principles of this disclosure, the magnetic performance of apermanent magnet may be improved by controlling crystallographicorientation (texture) of the grains in the microstructure of thepermanent magnet. This invention is especially powerful whencrystallographic texture control and magnetic orientation are combinedduring additive manufacturing. The result is a magnet which containsregions with different crystallographic and magnetic orientations, whichmay be optimized in various ways.

Some variations are predicated on favorable orientations of the magneticeasy axis within a magnet architecture. Preferred embodiments enable thecontrol of solidification of additively manufactured or weldedmicrostructures on the order of the single domain limit (e.g., about 1-3microns) to maximize the resistance to demagnetization in addition tocontrolling the orientation of the magnetic easy axis.

In some embodiments, easy axis alignment is designed into regions ofinterest, such as surfaces or corners, to improve overall resistance todemagnetization in a bulk magnet. This approach enables fabrication ofmagnets with a strong field on one side, while the field on the otherside is close to zero, for example. Such region optimization is on alength scale (e.g., less than 500 microns) that is infeasible withconventional manufacturing methods which require serial assembly.

The present invention increases permanent-magnet performance whilepotentially reducing manufacturing costs compared to currentmanufacturing methods described in the Background. Material costs ofrare-earth permanent-magnet materials, required for high-performanceautomotive and aerospace platforms, are significantly reduced by usingpowder-bed or spray-based additive manufacturing methods. The reason isthat a near-net-shape product can be produced with minimal materialwaste upon finishing and with a high rate of material recycling.

By improving the demagnetization resistance of magnetic architecturesthrough tailored crystallographic textures, the mass and volumeefficiency of magnets can be improved. This, in turn, reduces thenecessary material mass for matching performance to improve motorefficiencies. In addition, the invention provides the capability toproduce optimized magnet shapes which optimize field utility. Theability to use higher magnetic fields enables efficiency gains inpermanent-magnet motors. All of these factors improve permanent-magnetmotor efficiencies and decrease the overall cost to manufacture.

Some variations provide a permanent-magnet structure comprising:

a region having a plurality of magnetic domains and a region-averagemagnetic axis, wherein each of the magnetic domains has a domainmagnetic axis, wherein each domain magnetic axis is substantiallyaligned with the region-average magnetic axis, and wherein the pluralityof magnetic domains is characterized by an average magnetic domain size;and

within the region, a plurality of metal-containing grains, wherein theplurality of metal-containing grains is characterized by an averagegrain size,

wherein each of the magnetic domains has a domain easy axis that isdictated by a crystallographic texture of the metal-containing grains;

wherein the region has a region-average easy axis based on average valueof the domain easy axis within the region; and

wherein the region-average magnetic axis and the region-average easyaxis form a region-average alignment angle θ that has a θ standarddeviation of less than 30° based on alignment-angle variance within theplurality of magnetic domains.

A “permanent magnet” (or “hard magnet”) means a magnet with an intrinsicmagnetic coercivity of 1000 A/m (amperes per meter) or greater. Forexample, a permanent magnet may be selected from the group consisting ofa NdFeB magnet, a NdDyFeB magnet, a FeCoCr magnet, a FeAlNiCo magnet, aSmCo magnet, and combination thereof.

The “magnetic axis” is the straight line joining two poles of amagnetized body. The torque exerted on the magnet by a magnetic field inthe direction of the magnetic axis equals 0. The “crystallographictexture” is the distribution of crystallographic orientations of apolycrystalline material. A “crystallographic orientation” is defined bythe plane (Miller) indices of the lattice plane of a crystal.

There may be one region or many regions. When there are multipleregions, the individual region-average magnetic axes and the individualregion-average easy axes may vary spatially, such as differentorientations in different corners of the structure. Regions may be bulkregions contained in the interior of the permanent-magnet structureand/or surface regions contained at the surface of the permanent-magnetstructure.

The magnetic domain averages and easy axis averages may varyspatially—e.g., different orientations in different corners—in differentareas of the permanent-magnet structure. The variations across regionsmay be regular or irregular.

Each metal-containing grain has a grain easy axis. If a magnetic domainis the same as a grain, as in some embodiments, then the grain easy axisis the same as the domain easy axis. But if a magnetic domain is largerthan one grain (e.g., 5 grains), then the domain easy axis is dictatedby easy axes of all individual grains in the magnetic domain.

The metal-containing grains may contain a metal selected from the groupconsisting of Fe, Ni, Al, Co, Cr, Nd, B, Sm, Dy, and combinationsthereof. In some embodiments, the metal-containing grains contain ametal alloy selected from the group consisting of NdFeB, FeCoCr, AlNiCo,SmCo, Dy₂O₃, SrRuO₃, and combinations thereof. When there are multipleregions, there may be different compositions in those regions, includingdifferent types or amounts of metal-containing grains, for example. Anexample is a surface region with a different composition than a bulkregion of the permanent-magnet structure.

The selection of components in the overall composition will be dependenton the desired magnet properties and should be considered on acase-by-case basis. Someone skilled in the art of material science ormetallurgy will be able to select the appropriate materials for theintended use, based on the information provided in this disclosure.

In some embodiments, the region-average alignment angle θ is from −10°to 10°, from −5° to 5°, from −2° to 2°, or from −1° to 1°. In otherembodiments, the region-average alignment angle θ is selected from 0° to90°. In certain embodiments, the region-average alignment angle θ is 0°.In certain embodiments, the region-average alignment angle θ is 90°. Theregion-average alignment angle θ standard deviation may be less than20°, less than 10°, or less than 5°, for example.

In some embodiments, the region-average easy axis has a standarddeviation that is less than 25°, less than 20°, less than 10°, or lessthan 5°. This standard deviation is calculated based on all of thedomain easy axes within a given region. In certain embodiments, eachdomain easy axis is substantially aligned with the region-average easyaxis, in which case the standard deviation may be less than 2°, lessthan 1°, less than 0.5°, less than 0.1°, or about 0°.

A magnetic domain may contain an individual metal-containing grain.Typically, a magnetic domain contains multiple metal-containing grains.In some embodiments, the average magnetic domain size is about the sameas the average grain size. In other embodiments, the average magneticdomain size is larger than the average grain size.

The average magnetic domain size may be from 1 micron to 1000 microns,for example. In some embodiments, the average magnetic domain size isfrom 10 microns to 10 millimeters.

The average grain size may be from 0.1 microns to 50 microns, forexample. An exemplary average grain size is from about 1 micron to about5 microns.

Grain sizes may be measured by a variety of techniques, includingdynamic light scattering, laser diffraction, image analysis, or sieveseparation, for example. Dynamic light scattering is a non-invasive,well-established technique for measuring the size and size distributionof particles typically in the submicron region, and with the latesttechnology down to 1 nanometer. Laser diffraction is a widely usedparticle-sizing technique for materials ranging from hundreds ofnanometers up to several millimeters in size. Exemplary dynamic lightscattering instruments and laser diffraction instruments for measuringparticle sizes are available from Malvern Instruments Ltd.,Worcestershire, UK. Image analysis to estimate particle sizes anddistributions can be done directly on photomicrographs, scanningelectron micrographs, or other images.

In some embodiments, the metal-containing grains are substantiallyequiaxed grains. In other embodiments, the metal-containing grains aresubstantially columnar grains or elongated grains. In certainembodiments, the metal-containing grains are a combination ofsubstantially equiaxed grains and substantially columnar grains. Grainsmay be surrounded by a grain boundary layer with a different compositionand magnetic properties.

In the permanent-magnet structure, the region may have a characteristiclength scale selected from 100 microns to 1 meter, such as about 1 mm, 1cm, or 10 cm, for example.

In some permanent-magnet structures, there is at least one additionalregion having a plurality of additional magnetic domains and a pluralityof metal-containing grains. The additional region may have a differentcomposition compared to the bulk region(s).

In some embodiments, the additional region is contained at a corner oredge of the permanent-magnet structure. In these or other embodiments,the additional region is contained at a surface of the permanent-magnetstructure. For example, a corner or edge of a cuboid permanent magnetmay have crystallographic and magnetic orientation facing out of thecorner whereas the rest of the magnet has crystallographic and magneticorientation facing towards a bulk region internally.

In some embodiments, the permanent-magnet structure is contained withina Halbach array. Halbach arrays are conventionally assembled by bondingindividually uniform texture and magnetic orientation magnets in asequence of orientations that accentuates the field on one side of themagnet at the expense of the field on the opposing side. Thisconformation sacrifices field uniformity due to the large size (>1 mm)of the magnets used conventional Halbach arrays. By using the principlesdisclosed herein, a Halbach array configuration may be constructed atthe micron scale, thereby enabling more-uniform, high-flux magneticfields to be generated in the permanent magnet.

In some embodiments, the permanent-magnet structure is an additivelymanufactured structure. Additive manufacturing is discussed in moredetail below. In some embodiments, the permanent-magnet structure is awelded structure.

The permanent-magnet structure may be contained within a solid bulkmagnet. Alternatively, the permanent-magnet structure may be containedwithin a porous magnet. The porosity may vary, such as from 0% to about50%, or from 0% to about 20%, by volume. Additionally there may be sometopology including irregular surfaces, discontinuous interfaces,internal roughness, etc.

The present invention will now be further described, including withreference to the accompanying drawings that are not intended to limitthe scope of the invention. The drawings are not necessarily to scale.

FIG. 1 depicts a conventional isotropic magnet (left side) and aconventional anisotropic magnet (right side). In a conventionaldie-pressed isotropic magnet, crystallographic orientation of grains isnon-uniform and random, and magnetic alignment is along the samedirection throughout the volume of the magnet. In a conventionaldie-pressed anisotropic magnet, magnetic alignment is along the samedirection throughout the volume of the magnet, and there is somealignment and uniformity of crystallographic orientation of grains. Theanisotropic variant generates a higher-energy product due to thealignment along the easy axis.

In contrast, FIG. 2 depicts a solid, bulk magnet with a magnetic fieldapplied in three different directions within the three regions shown.The crystallographic orientation has not been controlled and istherefore not shown. In the permanent-magnet structure of FIG. 2, thereis location-specified magnetization orientation (but notlocation-specified crystallographic texture), in reference to the easyaxis orientation, throughout the total volume. The magnetic orientationchanges by 90° across each region from left to right in FIG. 2. Becausethe crystallographic orientation has not been controlled, it isstatistically unlikely to be aligned with the magnetic orientation. Thatis, in FIG. 2, a region-average magnetic axis and a region-average easyaxis form a region-average alignment angle θ that may have a high θstandard deviation (such as 30° or higher) based on alignment-anglevariance within the plurality of magnetic domains.

FIG. 3 depicts a solid, bulk magnet with magnetic field in threedifferent directions in the three regions shown. Additionally, thecrystallographic orientation has been controlled through thermalgradients in solidification (e.g., from additive manufacturing orwelding). In the permanent-magnet structure of FIG. 3, there islocation-specified magnetization orientation and crystallographictexture, in reference to the easy axis orientation, throughout the totalvolume. The magnetic orientation changes by 90° from the first (left)regions to the second (middle) region, and then changes by 180° from thesecond to the third (right) region in FIG. 3. The crystallographic easyaxis changes by approximately 90° across each regions from left to rightin FIG. 3. In the first two regions, the easy axis is closely alignedwith the magnetic orientation, resulting in increased energy product.

In each of FIG. 2 and FIG. 3, the height of the bulk magnet may be onthe order of 1 cm, while the individual grain size may be on the orderof 1 to 5 microns, for example. Thus the squares depicting multiplegrains and orientations represent a small portion of the volume of eachregion.

The difference between the structures in FIGS. 2 and 3 is that in FIG.2, magnetic orientation is controlled but the crystallographicorientation is not controlled, while in FIG. 3, both of the magneticorientation and the crystallographic orientation are controlled. Ineither case, the magnetization orientation may be designed with specificorientations in different regions (or voxels) of the permanent-magnetstructure. Magnetic domains may be larger than crystal grains. Incertain preferred embodiments, each individual grain is individuallytailorable, as noted in FIG. 3 (magnetic domains=crystal grains).

Control of both crystallographic texture and magnetization direction inpermanent magnets may utilize combinations of magnetization directionand easy axis alignment (grain orientations). This control may augmentmagnetic field distribution and tailor the field shape generated by thepermanent magnet. The voxel size of the controlled region may be 10μm×10 μm×10 μm in size, for example, dependent on theadditive-manufacturing, welding, or other method to control the localmicrostructure. In certain embodiments, single magnetic domains withuniform magnetic orientation exist in single grains which can beindividually oriented crystallographically.

Interfaces between domains are susceptible to domain reversal(demagnetization) due to a lower barrier to nucleation. By matching asingle magnetic domain with a single grain, the resistance to domainreversal within that grain is minimized, raising the energy barrier tonucleation of a reverse domain.

The local orientation of magnetization may be directionally tailored,even if unaligned with the easy axis, to shape the magnetic fieldgenerated by the permanent-magnet structure. This is depicted in FIG. 4which shows an exemplary permanent magnet with a shaped field due totailored magnetization, with north (N)-south (S) axis orientation havingangle θ from normal direction.

The permanent-magnet structure of FIG. 3 is a preferred structure due tolocation-specified magnetization orientation as well aslocation-specified crystallographic orientation. Location-specifiedmagnetization and crystallographic orientation may also be referred toas optimized, designated, or pre-selected orientations, since theorientations are designed into the structure intentionally, not randomlygenerated during fabrication.

Location-specified magnetization and crystallographic orientation,within various regions of the structure, may be optimized to account forthe anticipated use conditions. For example, designated orientations maybe desirable in locations of high susceptibility of demagnetization.These regions arise when demagnetizing field concentration is high orwhen orientations with respect to the field direction change rapidly,such as at corners.

An important example is the problem of demagnetization in electricmotors. The magnetization of permanent magnets is typically parallel tothe long surface. The applied magnetic flux density which magnetsexperience in electric motor applications is higher at the corners.Thus, the corners demagnetize easier, limiting performance. Also, atmagnet corners, edges, or surfaces, there can be severe local heatingthat raises the local temperature above the Curie temperature of themagnetic material. Normally this would mean that the material loses itspersistent magnetic field.

Conventionally, these problems are mitigated by shaping the magneticfield of the permanent magnets in an electric motor via arranging manysmaller magnets into a larger Halbach array and adhesively bonding thesmaller magnets together.

By contrast, demagnetization can be prevented or at least inhibited atcorners, edges, or surfaces by tailoring the crystallographic texture(easy axis alignment). Also, the local composition at the corners,edges, or surfaces that experience local heating may be optimized usinga different composition compared to the bulk region. For example, rareearth elements may be included, or at a higher concentration, atcorners, edges, or surfaces, or other additional regions compared to thebulk region.

Optimal easy axis orientations with respect to the external fieldpreferably increase the energetic barrier to the nucleation of a reversemagnetic domain. Such optimization preserves magnetization in higherapplied fields. In magnetocrystalline anisotropic materials, there mayexist more than one easy axis dependent on crystal structure whichdescribes the magneto-crystalline anisotropy. In the case of multipleeasy axes, the texture configuration may be chosen in any of theequivalent directions, which may assist in texture control.

The magnetization and crystallographic orientations may or may not beco-aligned in the magnet, even when both of these orientations arecontrolled. In some preferred embodiments, the magnetic orientation andcrystallographic orientation are co-aligned. The magnetization andcrystallographic orientations do not need to be co-aligned in the magnetto realize improved demagnetization benefits. In some embodiments, themagnetization and crystallographic orientations are controlled toachieve an average alignment angle between them.

FIG. 5 depicts several examples of magnetic configurations for differentorientations that may be employed. The macroscopic and microscopicmagnetic fields ({right arrow over (M)}) are only meant to distinguishtailored orientations at specific regions (corners and edges) comparedto the bulk region of each structure, without implying a necessarylength scale. Note also that while the arrows in FIG. 5 depict magneticorientations, similar structures may be drawn in which the arrows denotecrystallographic orientations, or multiple sets of arrows denotingmagnetic orientations along with crystallographic orientations (such asin FIG. 3).

The torque generated in two dimensions varies with (sin θ)² where themaximum torque, and resistance to demagnetization, is generatedperpendicular to the opposing magnetic field. The torque generated inthree dimensions, and the 3D resistance to demagnetization as a functionof position, may be modeled by one skilled in the art. 3D modeling maybe used to aid in pre-selecting regions of interest for tailoring thecrystallographic texture. For example, COMSOL Multiphysics® simulationsoftware may be utilized to computationally model and design apermanent-magnet structure.

The permanent-magnet structure is characterized by a total energyproduct that is the maximum of the magnetic remanence times magneticcoercivity for the structure. The permanent-magnet structure may have atotal energy product of greater than 50 kJ/m³, such as about, or atleast about, 100 kJ/m³, 200 kJ/m³, 300 kJ/m³, 400 kJ/m³, or 500 kJ/m³,including all intervening ranges. It is known to be difficult to attainhigh total energy product without using rare-earth metals. Although someembodiments herein incorporate rare-earth metals at least within certainregions of the structure, other embodiment incorporate no rare-earthmetals at all (except for impurities). In certain embodiments employingadditive manufacturing to control local solidification, thepermanent-magnet structure may have a total energy product of greaterthan 50 kJ/m³, such as about, or at least about, 75 kJ/m³, 100 kJ/m³,125 kJ/m³, 150 kJ/m³, 175 kJ/m³, or 200 kJ/m³, including all interveningranges. Tuned anisotropy may be utilized to get the same as, or closeto, rare-earth magnetic performance even if rare-earth metals are notincorporated at all or only at critical locations, such as corners.

The design of magnetic orientation and crystallographic orientationenables optimization of magnet properties in non-uniform demagnetizingfields present in nearly every application. In an opposing demagnetizingfield, magnetic orientations near the edges or surfaces ofpermanent-magnet structures become unaligned or at least becomesub-optimal in their orientations. By selectively manipulating the localorientation in these regions of rapidly demagnetizing field orientation,to instead resist demagnetization, the ceiling for operating conditions(e.g., temperature and field strength) can be raised. Performance can beimproved for demanding motor applications, especially electric vehiclepropulsion.

In some embodiments, magnetic domains are oriented to augment fields onone side of a structure, such as (but not limited to) a Halbach array. AHalbach array may be configured to cause cancellation of magneticcomponents, resulting in a one-sided magnetic flux, as depicted in FIG.6. A Halbach array is an arrangement of permanent magnets that augmentsthe magnetic field on one side of the array while cancelling the fieldto zero or near zero on the other side. An advantage of a one-sided fluxdistribution is that the field is twice as large on the side on whichthe flux is confined. Another advantage is the absence of a stray fieldon the opposite side, which helps with field confinement. Halbach arraysmay be used for brushless direct-current motors, free-electron lasers,and wiggler magnets for particle accelerators.

Some preferred variations of the invention will now be further describedin reference to additive manufacturing. It will be understood that theinvention is not limited to additive manufacturing, but additivemanufacturing is especially able to take advantage of many principlestaught herein.

Additive manufacturing provides control of crystallographic orientationduring 3D printing. The resultant crystallographic orientation of agrain is dependent on several contributing thermodynamic driving forces.One such factor is the direction of the maximum thermal gradient, inwhich solidifying cubic crystals tend to preferentially grow with a<100> orientation. The thermal gradient can be controlled using a laserscan strategy by locally heating with a variety of spatially and/ortemporally varying patterns. The formation of crystallographic texturecan also be tailored during solidification and subsequent solid-statetransformations through the application of an external magnetic field,potentially producing more texture uniformity with specified localityand direction. Tailorable magnetization may be achieved by varying localmagnetic coercivity when using laser or electron beam heat treatment.

In some embodiments, the magnetic performance is improved by controllingcrystallographic orientation (texture) of the grains in addition to themagnetization orientation in the microstructure. This control isespecially powerful when crystallographic texture and magneticorientation are both tailored in a synergistic way during additivemanufacturing. To this end, the grains may be crystallographicallyoriented in the (or a) direction that allows the highest remanentmagnetization; simultaneously, a magnetic field may be applied to orientthe magnetization in that same direction. The result is a magnet withoptimal crystallographic and magnetic orientation and therefore themaximum energy product. Magnetic materials may also be optimized by 3Dtailoring of crystallographic texture in regions susceptible todemagnetization. By employing additive manufacturing, local thermal,magnetic, and stress fields may be manipulated in the production ofpermanent magnets having selected crystallographic texture(s) withlocation specificity.

In some embodiments, without limitation, an additive-manufacturingfeedstock is a powder that is surface-functionalized with a plurality ofnanoparticles. The nanoparticles may promote heterogeneous nucleation inthe melt pool to induce equiaxed grain growth. In some embodiments, thenanoparticles are magnetic nanoparticles.

Some variations provide a method of making a permanent magnet withtailored magnetism, the method comprising:

(a) providing a feedstock composition containing one or more magnetic ormagnetically susceptible materials;

(b) exposing a first amount of the feedstock composition to an energysource for melting in a scan direction, thereby generating a first meltlayer;

(c) solidifying the first melt layer in the presence of an externallyapplied magnetic field, thereby generating a magnetic metal layercontaining a plurality of individual voxels;

(d) optionally repeating steps (b) and (c) a plurality of times togenerate a plurality of solid layers by sequentially solidifying aplurality of melt layers in a build direction, thereby generating aplurality of magnetic metal layers; and

(e) recovering a permanent magnet comprising the magnetic metal layer,

wherein the externally applied magnetic field has a magnetic-fieldorientation, defined with respect to the scan direction, that isselected to control (i) a magnetic axis within the magnetic metal layerand/or (ii) a crystallographic texture within the magnetic metal layer.

In some embodiments, the magnetic-field orientation is selected tocontrol the crystallographic texture but not necessarily the magneticaxis within the magnetic metal layer. In some embodiments, themagnetic-field orientation is selected to control the magnetic axis butnot necessarily the crystallographic texture within the magnetic metallayer. In preferred embodiments, the magnetic-field orientation isselected to control both the crystallographic texture as well as themagnetic axis within the magnetic metal layer.

The magnetic or magnetically susceptible material in the feedstockcomposition may include elemental metals, metal alloys, ceramics, metalmatrix composites, or combinations thereof, for example. The feedstockcomposition may be in the form of a powder, a wire, or a combinationthereof, for example.

The magnetic or magnetically susceptible material may be selected fromthe group consisting of Fe, Co, Ni, Cu, Cr, Mg, Mn, Zn, Sr, Ce, Si, B,C, Ba, Tb, Pr, Sm, Nd, Dy, Gd (gadolinium), and combinations or alloysthereof. Exemplary alloys that are magnetic or magnetically susceptibleinclude, but are not limited to, Fe₂O₃, FeSi, FeNi, FeZn, MnZn, NdFeB,NdDyFeB, FeCoCr, FeAlNiCo, AlNiCo, SmCo, Dy₂O₃, SrRuO₃, and combinationsthereof.

In some embodiments, without limitation, an additive-manufacturingfeedstock is a powder that is surface-functionalized with a plurality ofnanoparticles. The nanoparticles may promote heterogeneous nucleation inthe melt pool to induce equiaxed grain growth. In some embodiments, thenanoparticles are magnetic or magnetically susceptible and becomemagnetically aligned during solidification to produce a crystallographictexture dictated by an external magnetic field. Alternatively, oradditionally, the nanoparticles—whether or not they are magnetic ormagnetically susceptible—may induce growth of a magnetic phase whichcould then be magnetically aligned with the magnetic-field direction.

A magnetically susceptible material is a material that will becomemagnetized in an applied magnetic field. Magnetic susceptibilityindicates whether a material is attracted into or repelled out of amagnetic field. Paramagnetic materials align with the applied field andare attracted to regions of greater magnetic field. Diamagneticmaterials are anti-aligned and are pushed toward regions of lowermagnetic fields. On top of the applied field, the magnetization of thematerial adds its own magnetic field. The magnetizability of materialsarises from the atomic-level magnetic properties of the particles ofwhich they are made, typically being dominated by the magnetic momentsof electrons.

The energy source may be a laser beam, an electron beam, or both a laserbeam and an electron beam. The energy source preferably imposes athermal gradient that melts a portion of the feedstock composition in ascan direction, rather than bulk melting the entire feedstockcomposition. In some embodiments, steps (b) and (c) utilize a techniqueselected from the group consisting of selective laser melting, electronbeam melting, laser engineered net shaping, selective laser sintering,direct metal laser sintering, integrated laser melting with machining,laser powder injection, laser consolidation, direct metal deposition,directed energy deposition, plasma arc-based fabrication, ultrasonicconsolidation, electric arc melting, and combinations thereof.

In some embodiments, step (b) is also conducted in the presence of theexternally applied magnetic field, along with step (c). Optionally, themagnetic-field orientation may be adjusted during step (b).

Steps (b) and (c) together may be referred to as additive manufacturingor welding. When step (d) is employed to generate a plurality of solidlayers by sequentially solidifying a plurality of melt layers in a builddirection, then steps (b) and (c) together are typically referred to asadditive manufacturing or 3D printing.

When step (d) is conducted, the magnetic-field orientation may beadjusted in the build direction. The magnetic-field orientation may beadjusted at every build layer, or may switch back and forth between twodifferent orientations for successive layers, or may incrementallychange angle as the build proceeds, as just a few examples of buildstrategies.

In some preferred embodiments, the magnetic-field orientation isadjusted during step (c), i.e. during solidification of the first meltlayer. For example, after some voxels have been formed in a first meltlayer, the magnetic-field orientation may be adjusted, after which morevoxels are formed in the first melt layer.

In some embodiments, the magnetic-field orientation is selected tocontrol voxel-specific magnetic axes within the plurality of individualvoxels contained within the magnetic metal layer. In these or otherembodiments, the magnetic-field orientation is selected to controlvoxel-specific crystallographic textures within the plurality ofindividual voxels contained within the magnetic metal layer.

A “voxel” is a volumetric (3D) pixel. A plurality of voxels forms asingle layer having a thickness defined by the voxel height. In someembodiments, the individual voxels are defined by a characteristic voxellength scale selected from about 50 microns to about 1000 microns. Incertain embodiments, the characteristic voxel length scale is selectedfrom about 100 microns to about 500 microns. An exemplary voxel is onthe order of 100 μm×100 μm×100 μm. Another exemplary voxel is on theorder of 10 μm×10 μm×10 μm.

A voxel may be cubic in geometry, but that is not necessary. Forexample, a voxel may be rectangular or may have an irregular shape. Foran arbitrary voxel geometry, there is a characteristic voxel lengthscale that is equivalent to the cube root of the average voxel volume.The characteristic voxel length scale may be about, at least about, orat most about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 50, 75,100, 125, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, or1000 microns, including all intervening ranges (e.g., 100-500 microns).The characteristic voxel length scale is typically a function of thelaser or electron beam intensity, beam diameter, scan speed, andproperties (e.g., kinematic viscosity) of the material being fabricated.

In preferred embodiments utilizing additive manufacturing, there issolidification of individual voxels and the magnetic field may be variedvoxel-by-voxel, if desired. Using a highly localized energy source, andpotentially using different compositions during fabrication, smallvoxels of a structure can be created with specific crystal orientationsand magnetic properties, independently of other voxels.

Depending on the intensity of the energy delivered, each voxel may becreated by melting and solidification of a starting feedstock or bysintering or other heat treatment of a region of material, for example.During solidification, a molten form of a voxel produces one or moresolid grains with individual crystal structures. In some embodiments,solidified voxels contain single grains. In other embodiments,solidified voxels contain a plurality of grains having some distributionof crystallographic orientations and magnetic orientations.Geometrically, an individual voxel may be the same size as an individualgrain, or may be larger than an average grain size within a magneticmetal layer. In various embodiments, an average voxel contains about, atleast about, or at most about 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9, 10,15, 20, 25, 50, 75, or 100 grains, including all intervening ranges.

When a voxel contains a plurality of grains each having its owncrystallographic orientation and magnetic easy axis, the voxel will havea voxel-average crystallographic orientation and a voxel-averagemagnetic easy axis. In some embodiments, a voxel is configured such thatall grains have the same or similar crystallographic orientations and/ormagnetic easy axes. In other embodiments, a voxel is configured suchthat individual grains have different crystallographic orientationsand/or magnetic easy axes.

A magnetic metal layer from additive manufacturing or welding hascrystallographic texture arising from individual grains which, in turn,form voxels. There is a magnetic easy axis for each grain, an averagemagnetic easy axis for each voxel, and an average magnetic easy axis forthe magnetic metal layer. Using the principles of this disclosure, theremay be varying degrees of alignment between these hierarchical magneticeasy axes.

In certain embodiments, a voxel contains a plurality of grains with anarrow crystallographic orientation distribution along the easy axis ofthe crystal as well as co-aligned magnetic domains contained within eachgrain. This co-alignment produces the maximum total remanent magneticflux for the voxel. In a larger structure with a plurality of voxels,there may be a narrow crystallographic orientation distribution alongthe easy axis as well as co-aligned magnetic domains contained withineach voxel. This co-alignment produces the maximum total remanentmagnetic flux for the structure.

The grain sizes may vary widely, such as from about 0.1 microns to about1000 microns. In various embodiments, the average grain size (within agiven voxel or within the overall structure) may be about, at leastabout, or at most about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1,1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 75,100, 125, 150, 200, 250, 300, 350, 400, 500, 600, 700, 800, 900, or 1000microns.

When step (d) is conducted, to generate a plurality of solid layers bysequentially solidifying a plurality of melt layers in a builddirection, the magnetic-field orientation may be adjusted in the builddirection. In other words, the magnetic-field orientation may bedifferent for one layer versus another layer, in addition to variationsof the magnetic-field orientation within a layer (voxel-specificmagnetic-field orientations).

The atomic structure of a crystal introduces preferential directions formagnetization. This is referred to as magnetocrystalline anisotropy. A“magnetic easy axis” is a direction inside a crystal, along which asmall applied magnetic field is sufficient to reach the saturationmagnetization. There can be a single easy axis or multiple easy axes. A“magnetic hard axis” is a direction inside a crystal, along which alarge applied magnetic field is needed to reach the saturationmagnetization. There will be a magnetic easy axis and a magnetic hardaxis whether or not a magnetic field is actually being applied. Themagnetic easy axis is different from the magnetic axis. A magnetic axisis only present when a magnetic field is actually applied, whereas amagnetic easy axis is a fixed property of a given crystalline material.

In some embodiments of the invention, the magnetic-field orientation maybe selected to control voxel-specific magnetic easy axes within theplurality of individual voxels contained within the magnetic metallayer.

In some embodiments, the individual voxels are substantiallymagnetically aligned with each other, in reference to the magnetic easyaxes of each voxel within a given magnetic metal layer. By“substantially magnetically aligned” is it meant that there is astandard deviation that is less than 25°, less than 20°, less than 10°,or less than 5°, calculated based on all of the magnetic easy axeswithin the magnetic metal layer. In certain embodiments, all magneticeasy axes are substantially aligned, such that the standard deviation isless than 2°, less than 1°, less than 0.5°, less than 0.1°, or about 0°.Remanence measurements may be used to determine the alignment ofmagnetic easy axes. See McCurrie, “Determination of the degree of easyaxis alignment in uniaxial permanent magnets from remanencemeasurements” Journal of Applied Physics 52, 7344 (1981), which ishereby incorporated by reference.

In some preferred embodiments, the magnetic-field orientation isselected to control voxel-specific magnetic axes as well asvoxel-specific magnetic easy axes within the plurality of individualvoxels contained within the magnetic metal layer, wherein thevoxel-specific magnetic axes are substantially aligned with thevoxel-specific magnetic easy axes for at least a portion of the magneticmetal layer. In certain embodiments, the voxel-specific magnetic axesare substantially aligned with the voxel-specific magnetic easy axes forall of the magnetic metal layer.

In other embodiments, the magnetic-field orientation is selected tocontrol voxel-specific magnetic axes as well as voxel-specific magneticeasy axes within the plurality of individual voxels contained within themagnetic metal layer, wherein the voxel-specific magnetic axes areconfigured to be at angles with the voxel-specific magnetic easy axesfor at least a portion of the magnetic metal layer.

In some methods, conditions in step (b) and/or step (c) are controlledsuch that thermal gradients assist in generating the crystallographictexture within the magnetic metal layer.

In some embodiments, different feedstock compositions, each comprisingone or more magnetic or magnetically susceptible materials, are exposedto the energy source. The crystallographic texture may be adjustedduring the method by performing step (b), step (c), and optionally step(d) at different times using different feedstock compositions. Differentfeedstock compositions may be not only different species, but alsodifferent concentrations of the same species.

Optionally, different feedstock compositions, each comprising one ormore magnetic or magnetically susceptible surface-modifying particles,are exposed to the energy source, and the crystallographic texture isadjusted during the method by performing step (b), step (c), andoptionally step (d) at different times using the different feedstockcompositions.

In certain embodiments, neither the base particles nor thesurface-modifying particles are magnetic or magnetically susceptible,but during fabrication, grains are produced which are magnetic ormagnetically susceptible. An externally applied magnetic field may alignthose grains during solidification.

Different feedstock compositions also enable the fabrication of gradedcompositions. For instance, the concentration of magnetic rare earthelements may be adjusted throughout a magnetic structure. One exampleemploys local doping of Dy, Nd, or Yb in areas that are susceptible todemagnetization. Local doping may be achieved via spray additiveprocesses, for example.

Some embodiments optimize the crystallographic texture site-specificallythroughout the volume of the magnet. In contrast to conventionallyprocessed magnetic materials with a single easy axis orientation, or anarrow distribution of easy axis orientations, texture-controlledpermanent magnets disclosed herein may possess easy axis orientationstailored to resist demagnetizing fields in regions of highsusceptibility of demagnetization. Such regions may exist wheredemagnetizing field concentration is high and/or where orientations withrespect to the magnetic field direction change rapidly, such as atcorners.

Interfaces between domains are susceptible to domain reversal(demagnetization) due to a relatively low barrier to nucleation. Optimaleasy axis orientations with respect to the external magnetic fieldpreferably increase the energy barrier to nucleation of a reversemagnetic domain, thereby preserving magnetization. For example, bymatching a single magnetic domain with a single grain, the resistance todomain reversal within that grain is minimized, raising the energybarrier to nucleation of a reverse domain.

In general, the geometry of the feedstock composition is not limited andmay be, for example, in the form of powder particles, wires, rods, bars,plates, films, coils, spheres, cubes, prisms, cones, irregular shapes,or combinations thereof. In certain embodiments, the feedstockcomposition is in the form of a powder, a wire, or a combination thereof(e.g., a wire with powder on the surface). When the feedstockcomposition is in the form of powder, the powder particles may have anaverage diameter from about 1 micron to about 500 microns, such as about10 microns to about 100 microns, for example. When the feedstockcomposition is in the form of a wire, the wire may have an averagediameter from about 10 microns to about 1000 microns, such as about 50microns to about 500 microns, for example.

The energy source for additive manufacturing may be provided by a laserbeam, an electron beam, alternating current, direct current, plasmaenergy, induction heating from an applied magnetic field, ultrasonicenergy, other sources, or a combination thereof. Typically, the energysource is a laser beam or an electron beam.

Process steps (b) and (c) may utilize a technique selected from thegroup consisting of selective laser melting, electron beam melting,laser engineered net shaping, selective laser sintering, direct metallaser sintering, integrated laser melting with machining, laser powderinjection, laser consolidation, direct metal deposition, wire-directedenergy deposition, plasma arc-based fabrication, ultrasonicconsolidation, and combinations thereof, for example.

In certain embodiments, the additive manufacturing process is selectedfrom the group consisting of selective laser melting, energy-beammelting, laser engineered net shaping, and combinations thereof.

Selective laser melting utilizes a laser (e.g., Yb-fiber laser) toprovide energy for melting. Selective laser melting is designed to use ahigh power-density laser to melt and fuse metallic powders together. Theprocess has the ability to fully melt the metal material into a solid 3Dpart. A combination of direct drive motors and mirrors, rather thanfixed optical lens, may be employed. An inert atmosphere is usuallyemployed. A vacuum chamber can be fully purged between build cycles,allowing for lower oxygen concentrations and reduced gas leakage.Selective laser melting is a type of powder bed-based additivemanufacturing.

Electron beam melting uses a heated powder bed of metal that is thenmelted and formed layer by layer, in a vacuum, using an electron beamenergy source similar to that of an electron microscope. Metal powder iswelded together, layer by layer, under vacuum. Electron beam melting isanother type of powder bed-based additive manufacturing.

Laser engineering net shaping is a powder-injected technique operates byinjecting metal powder into a molten pool of metal using a laser as theenergy source. Laser engineered net shaping is useful for fabricatingmetal parts directly from a computer-aided design solid model by using ametal powder injected into a molten pool created by a focused,high-powered laser beam. Laser engineered net shaping is similar toselective laser sintering, but the metal powder is applied only wherematerial is being added to the part at that moment. Note that “netshaping” is meant to encompass “near net” fabrication.

Direct metal laser sintering process works by melting fine powders ofmetal in a powder bed, layer by layer. A laser supplies the necessaryenergy and the system operates in a protective atmosphere, typically ofnitrogen or argon.

Another approach utilizes powder injection to provide the material to bedeposited. Instead of a bed of powder that is reacted with an energybeam, powder is injected through a nozzle that is then melted to depositmaterial. The powder may be injected through an inert carrier gas or bygravity feed. A separate shielding gas may be used to protect the moltenmetal pool from oxidation.

Directed energy deposition utilizes focused energy (either an electronbeam or laser beam) to fuse materials by melting as the material isbeing deposited. Powder or wire feedstock can be utilized with thisprocess. Powder-fed systems, such as laser metal deposition and laserengineered net shaping, blow powder through a nozzle, with the powdermelted by a laser beam on the surface of the part. Laser-based wirefeedsystems, such as laser metal deposition-wire, feed wire through a nozzlewith the wire melted by a laser, with inert gas shielding in either anopen environment (gas surrounding the laser), or in a sealed gasenclosure or chamber.

Powder bed-based additive manufacturing is preferred for its ability toproduce near-net-shape products as well as the smaller tailorable voxelsize (such as about 200 μm or less) compared to directed energydeposition (conventionally >500 μm).

Some embodiments utilize wire feedstock and an electron beam heat sourceto produce a near-net shape part inside a vacuum chamber. An electronbeam gun deposits metal via the wire feedstock, layer by layer, untilthe part reaches the desired shape. Then the part optionally undergoesfinish heat treatment and machining. Wire can be preferred over powderfor safety and cost reasons.

Additive manufacturing provides the opportunity to tailor localstructure voxel-by-voxel in a serial, layered process. A processed voxelis the volume affected by heat input from the direct energy source in alayer-based approach, which volume includes the melt pool as well as thesurrounding heat-affected zone. The solidification crystallographictexture may be controlled by the direction of heat extraction. Inaddition to the thermal field, a magnetic field may be applied duringprocessing to control both crystallographic texture and magnetizationorientation. The external magnetic field may be generated by means of aninduction coil, multiple induction coils, a permanent magnet, or anarray of permanent magnets, for example.

In some embodiments, the additively manufactured permanent magnet has amicrostructure with a crystallographic texture that is not solelyoriented in the additive-manufacturing build direction. For example, thesolid layers may have differing primary growth-direction angles withrespect to each other.

The method is not limited in principle to the number of solid layersthat may be fabricated. A “plurality of solid layers” in step (d) meansat least 2 layers, such as at least 10 individual solid layers. Thenumber of solid layers may be much greater than 10, such as about 100,1000, or more. As noted earlier, in the case of welding or single-layermanufacturing, there may be a single layer in the final structure.Multiple-layer welding is another embodiment.

The plurality of solid layers may be characterized by an average layerthickness of at least 10 microns, such as about 10, 20, 30, 40, 50, 75,100, 150, or 200 microns, for example.

Each solid layer may contain a number of voxels. In a special case for asubstantially vertical build (e.g., a narrow column), there may be asingle voxel per layer. The average number of voxels per layer may beabout, at least about, or at most about 2, 3, 4, 5, 10, 20, 30, 40, 50,100, 150, 200, 300, 400, 500, 600, 700, 800, 900, or 1000, including allintervening ranges, for example.

One or more solid layers may have a microstructure with equiaxed grains.A microstructure that has “equiaxed grains” means that at least 90 vol%, preferably at least 95 vol %, and more preferably at least 99 vol %of the metal alloy contains grains that are roughly equal in length,width, and height. In some embodiments, at least 99 vol % of the magnetcontains grains that are characterized in that there is less than 25 %,preferably less than 10 %, and more preferably less than 5 % standarddeviation in each of average grain length, average grain width, andaverage grain height. Equiaxed grains may result when there are manynucleation sites arising from grain-refining nanoparticles contained inthe microstructure.

The surface-modifying particles of some embodiments are grain-refiningnanoparticles. The grain-refining nanoparticles are preferably presentin a concentration of at least 0.01 vol %, such as at least 0.1 vol %,at least 1 vol %, or at least 5 vol % of the feedstock composition. Invarious embodiments, the grain-refining nanoparticles are present in aconcentration of about, or at least about, 0.1, 0.2, 0.5, 1, 2, 3, 4, 5,6, 7, 8, 9, or 10 vol %.

In some embodiments, the grain-refining nanoparticles arelattice-matched to within ±5% compared to an otherwise-equivalentmaterial containing the base particles but not the grain-refiningnanoparticles. In certain embodiments, the grain-refining nanoparticlesare lattice-matched to within ±2% or within ±0.5% compared to a materialcontaining the base particles but not the grain-refining nanoparticles.

Preferably, the microstructure of the additively manufactured magnet issubstantially crack-free. The avoidance of cracks can be important formagnets. For example, samarium-cobalt magnets are brittle and prone tocracking and chipping. Crack-free SmCo-based permanent magnets may befabricated.

A magnet microstructure that is “substantially crack-free” means that atleast 99.9 vol % of the metal alloy contains no linear or tortuouscracks that are greater than 0.1 microns in width and greater than 10microns in length. In other words, to be considered a crack, a defectmust be a void space that is at least 0.1 microns in width as well as atleast 10 microns in length. A void space that has a length shorter than10 microns but larger than 1 micron, regardless of width, can beconsidered a porous void (see below). A void space that has a length ofat least 10 microns but a width shorter than 0.1 microns is amolecular-level gap that is not considered a defect.

Typically, a crack contains open space, which may be vacuum or maycontain a gas such as air, CO₂, N₂, and/or Ar. A crack may also containsolid material different from the primary material phase of the metalalloy. The non-desirable material disposed within the crack may itselfcontain a higher porosity than the bulk material, may contain adifferent crystalline (or amorphous) phase of solid, or may be adifferent material altogether, arising from impurities duringfabrication, for example.

The magnet microstructure may be substantially free of porous defects,in addition to being substantially crack-free. “Substantially free ofporous defects” means at least 99 vol % of the magnet contains no porousvoids having an effective diameter of at least 1 micron.

Preferably, at least 80 vol %, more preferably at least 90 vol %, evenmore preferably at least 95 vol %, and most preferably at least 99 vol %of the magnet contains no porous voids having an effective diameter ofat least 1 micron. A porous void that has an effective diameter lessthan 1 micron is not typically considered a defect, as it is generallydifficult to detect by conventional non-destructive evaluation. Alsopreferably, at least 90 vol %, more preferably at least 95 vol %, evenmore preferably at least 99 vol %, and most preferably at least 99.9 vol% of the metal alloy contains no larger porous voids having an effectivediameter of at least 5 microns.

Typically, a porous void contains open space, which may be vacuum or maycontain a gas such as air, CO₂, N₂, and/or Ar. Porous voids may bereduced or eliminated, in some embodiments. For example, additivelymanufactured metal parts may be hot-isostatic-pressed to reduce residualporosity, and optionally to arrive at a final additively manufacturedmagnet that is substantially free of porous defects in addition to beingsubstantially crack-free.

In additive manufacturing, post-production processes such as heattreatment, light machining, surface finishing, coloring, stamping, orother finishing operations may be applied. Also, several additivemanufactured parts may be joined together chemically or physically toproduce a final magnet.

EXAMPLE

This example demonstrates crystallographic texture control of alaser-welded Nd₂Fe₁₄B magnet structure.

A Nd₂Fe₁₄B magnet structure is processed in a laser-welding machine withan external magnetic field. A DC magnetic field with flux density ofapproximately 1 T is applied parallel to the horizontal face of asintered Nd₂Fe₁₄B magnet structure. Single-track weld lines aregenerated using a pulsed infrared laser-welder moving the surfacethrough the static DC field.

FIG. 7 shows photomicrographs of the Nd₂Fe₁₄B magnet microstructure witha shifted dendrite growth direction against the direction of maximumthermal gradient. FIG. 8 shows a photomicrograph top view of thelaser-welded Nd₂Fe₁₄B magnet structure. FIG. 9 shows the increase inNdFeB easy axis [001] texture along the scan vector direction in IPF(inverse pole figure) density plots. EBSD (electron backscatterdiffraction) maps are provided for reference orientations. The IPFdensity plots are shown in the lower half of FIG. 9, and the EBSD mapsare shown in the upper half of FIG. 9.

Epitaxial dendritic solidification tends to grow in the direction of thelargest thermal gradient. However, under an external magnetic field, thesolidification direction is unaligned with the maximum thermal gradient.Adjustment of the field direction influences texture evolution (from thedendritic solidification direction) with greater or less magnitude inaccordance with the magnitude and field applied.

In this detailed description, reference has been made to multipleembodiments and to the accompanying drawings in which are shown by wayof illustration specific exemplary embodiments of the invention. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatmodifications to the various disclosed embodiments may be made by askilled artisan.

Where methods and steps described above indicate certain eventsoccurring in certain order, those of ordinary skill in the art willrecognize that the ordering of certain steps may be modified and thatsuch modifications are in accordance with the variations of theinvention. Additionally, certain steps may be performed concurrently ina parallel process when possible, as well as performed sequentially.

All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each publication, patent, or patent application were specifically andindividually put forth herein.

The embodiments, variations, and figures described above should providean indication of the utility and versatility of the present invention.Other embodiments that do not provide all of the features and advantagesset forth herein may also be utilized, without departing from the spiritand scope of the present invention. Such modifications and variationsare considered to be within the scope of the invention defined by theclaims.

What is claimed is:
 1. A permanent-magnet structure comprising: a regionhaving a plurality of magnetic domains and a region-average magneticaxis, wherein each of said magnetic domains has a domain magnetic axis,wherein each said domain magnetic axis is substantially aligned withsaid region-average magnetic axis, and wherein said plurality ofmagnetic domains is characterized by an average magnetic domain size;and within said region, a plurality of metal-containing grains, whereinsaid plurality of metal-containing grains is characterized by an averagegrain size, wherein each of said magnetic domains has a domain easy axisthat is dictated by a crystallographic texture of said metal-containinggrains; wherein said region has a region-average easy axis based onaverage value of said domain easy axis within said region; and whereinsaid region-average magnetic axis and said region-average easy axis forma region-average alignment angle θ that has a θ standard deviation ofless than 30° based on alignment-angle variance within said plurality ofmagnetic domains.
 2. The permanent-magnet structure of claim 1, whereinsaid metal-containing grains contain a metal alloy selected from thegroup consisting of NdFeB, FeCoCr, FeAlNiCo, SmCo, Dy₂O₃, SrRuO₃, andcombinations thereof.
 3. The permanent-magnet structure of claim 1,wherein said θ standard deviation is less than 20°.
 4. Thepermanent-magnet structure of claim 1, wherein said θ standard deviationis less than 10°.
 5. The permanent-magnet structure of claim 1, whereinsaid θ standard deviation is less than 5°.
 6. The permanent-magnetstructure of claim 1, wherein said region-average alignment angle θ isfrom −10° to 10°.
 7. The permanent-magnet structure of claim 1, whereinsaid region-average alignment angle θ is from −2° to 2°.
 8. Thepermanent-magnet structure of claim 1, wherein said region-averagealignment angle θ is selected from 0° to 90°.
 9. The permanent-magnetstructure of claim 1, wherein said region-average alignment angle θ is0°.
 10. The permanent-magnet structure of claim 1, wherein saidregion-average alignment angle θ is 90°.
 11. The permanent-magnetstructure of claim 1, wherein said region-average easy axis has astandard deviation that is less than 25°.
 12. The permanent-magnetstructure of claim 1, wherein said region-average easy axis has astandard deviation that is less than 20°.
 13. The permanent-magnetstructure of claim 1, wherein said region-average easy axis has astandard deviation that is less than 10°.
 14. The permanent-magnetstructure of claim 1, wherein said region-average easy axis has astandard deviation that is less than 5°.
 15. The permanent-magnetstructure of claim 1, wherein each said domain easy axis issubstantially aligned with said region-average easy axis.
 16. Thepermanent-magnet structure of claim 1, wherein said average magneticdomain size is about the same as said average grain size.
 17. Thepermanent-magnet structure of claim 1, wherein said average magneticdomain size is larger than said average grain size.
 18. Thepermanent-magnet structure of claim 1, wherein said average magneticdomain size is from 1 micron to 1000 microns.
 19. The permanent-magnetstructure of claim 1, wherein said average magnetic domain size is from10 microns to 10 millimeters.
 20. The permanent-magnet structure ofclaim 1, wherein said average grain size is from 0.1 microns to 50microns.
 21. The permanent-magnet structure of claim 1, wherein saidmetal-containing grains are substantially equiaxed grains.
 22. Thepermanent-magnet structure of claim 1, wherein said metal-containinggrains are substantially columnar grains.
 23. The permanent-magnetstructure of claim 1, wherein said metal-containing grains are acombination of substantially equiaxed grains and substantially columnargrains.
 24. The permanent-magnet structure of claim 1, wherein saidregion has a characteristic length scale selected from 100 microns to 1meter.
 25. The permanent-magnet structure of claim 1, wherein saidpermanent-magnet structure contains at least one additional regionhaving a plurality of additional magnetic domains and a plurality ofmetal-containing grains.
 26. The permanent-magnet structure of claim 25,wherein said additional region is contained at a corner or edge of saidpermanent-magnet structure.
 27. The permanent-magnet structure of claim25, wherein said additional region is contained at a surface of saidpermanent-magnet structure.
 28. The permanent-magnet structure of claim25, wherein said additional region has a different composition comparedto said region.
 29. The permanent-magnet structure of claim 1, whereinsaid permanent-magnet structure is contained within a Halbach array. 30.The permanent-magnet structure of claim 1, wherein said permanent-magnetstructure is an additively manufactured structure.
 31. Thepermanent-magnet structure of claim 1, wherein said permanent-magnetstructure is a welded structure.
 32. The permanent-magnet structure ofclaim 1, wherein said permanent-magnet structure is contained within asolid bulk magnet.
 33. The permanent-magnet structure of claim 1,wherein said permanent-magnet structure is contained within a porousmagnet.